Changes of estrous cycles with aging in female F344/n rats.

Changes of estrous cycles with aging of F344/N rats between 1 and 30 months of age (M) were monitored by vaginal smear cytology. The vaginal opening and first cornified cell phase were identified at 1.3 +/- 0.1 M and 1.5 +/- 0.2 M, respectively. Thereafter, estrous cycles showed about 5-day intervals, and ceased at 16.4 +/- 1.2 M. Thereafter irregular appearance of single cornified cell phases without the preceding of nucleated cell phases interspersed with a predominant leukocyte phase was seen in vaginal smears until 26.9 +/- 0.5 M. Growing and mature follicles as well as corpora lutea persisted until at least 30 M, and characterized the post reproductive aging of F344/N females. The F344/N rats seem to resemble humans in that the cessation of estrous cycles occurs at approximately half their entire lifespan. However, other aging characteristics are unknown in postmenopausal women. Therefore, we must be careful when extrapolating the aging changes of reproduction in F344/N rats to human beings.